1. Field of the Invention
The present invention relates to an exhaust purification apparatus for an internal combustion engine, which apparatus includes air-fuel-ratio sensors disposed in an exhaust passage of the engine to be located upstream and downstream, respectively, of a catalyst unit disposed in the exhaust passage, and feedback-controls the air-fuel ratio of the engine on the basis of respective outputs of the sensors.
2. Description of the Related Art
For example, Japanese Patent Application Laid-Open (kokai) No. 7-197837 discloses a conventional exhaust purification apparatus of such a type. The disclosed exhaust purification apparatus for an internal combustion engine includes an upstream air-fuel-ratio sensor and a downstream air-fuel-ratio sensor, which are disposed upstream and downstream, respectively, of a catalyst unit disposed in an exhaust passage of the engine. A sub-feedback control quantity is calculated on the basis of (through proportional plus integral plus derivative processing (PID processing) of a deviation, from a predetermined downstream-side target value, of the output value of the downstream air-fuel-ratio sensor. Meanwhile, a main-feedback control quantity is calculated on the basis of (through proportional plus integral processing (PI processing) on a deviation, from a predetermined upstream-side target value, of the output value of the upstream air-fuel-ratio sensor having been corrected by use of the sub-feedback control quantity. The air-fuel ratio of the engine is feedback-controlled through correction of fuel injection quantity on the basis of the main-feedback control quantity.
The disclosed apparatus is configured in such a manner that the above-described deviation used to calculate the main-feedback control quantity is directly changed in accordance with the value of the sub-feedback control quantity. In other words, a main-feedback controller, which performs main-feedback control (calculates the main-feedback control quantity), and a sub-feedback controller, which performs sub-feedback control (calculates the sub-feedback control quantity), are arranged in series.
Accordingly, when one of a main-feedback control constant (proportional gain or integral gain) used by the main-feedback controller and a sub-feedback control constant (proportional gain, integral gain, or derivative gain) used by the sub-feedback controller is adjusted for optimization, the value of the other constant greatly affects the adjustment. In other words, adjustment of the feedback control constants of the main-feedback control loop (closed loop) and adjustment of the feedback control constants of the sub-feedback control loop (closed loop) cannot be performed independently of each other. Accordingly, adjustment of the feedback control constants has conventionally been difficult and required much labor.